Removal of the surface layers of human cortical bone allografts restores in vitro osteoclast function reduced by processing and frozen storage.

A major complication of cortical bone grafting is nonunion at the host-graft junction. Many factors are thought to be involved in successful engraftment including the quality of the graft and the host response to it. In particular, the recipient osteoclasts (OCs) play a critical role by resorbing the engrafted bone. Thus, effective engraftment may depend on the inherent biological properties of the bone graft, which subsequently correlates with early and effective OC resorption. Normally, bone grafts are stored and processed by freezing, freeze-drying, irradiation, and lipid extraction. We sought to determine whether processing and storage affected bone quality, as evaluated by OC bone resorption. Cortical bone specimens from six human donors were either fresh, frozen at −75°C, or had undergone combinations of freezing at −75°C, freeze-drying, lipid extraction, irradiation, and treatment with hydrogen peroxide. Bone slices of 0.5-mm thickness taken from the surface, beneath the surface, and at a depth of 7.5 mm were incubated with isolated rabbit OCs and resorption lacunae were measured. We observed highest OC activity with fresh bone followed by frozen, partially processed, and fully processed bone. When allografts were stored at −75°C for 12 months, there was up to a 4.2-fold reduction in OC activity on the surface layer. Additionally, we found reduced OC activity upon the outer surface bone compared to the inner layers. Removal of more than 0.5 mm of frozen and processed bone significantly improved OC activity. These results imply that inner bone layers of stored and processed bone allografts are protected against degradation of bone matrix components, except when frozen for extended periods of time. Taken together, these data suggest that bone allografts should be stored for less than 1 year and require the removal of at least 0.5 mm from their surface prior to transplantation.